This invention pertains to vehicular mounting or cushioning assemblies involving a resilient rubber or elastomeric body that is adhesively bonded to a bracket or containment member.
There are several applications in automobile technology in which a cushioning or mounting member is employed to support an engine or a transmission to body members or to provide a cushioned connection (e.g., a bushing) between suspension members. A typical engine mount or transmission mount, for example, employs a resilient body of polyisoprene rubber, or such rubber mixed with other suitable elastomer material, sandwiched, sometimes under pressure, between cooperating bracket members. One of the bracket members is connected to the engine or to the transmission, and another bracket member is attached to a vehicle body member. In addition to being sandwiched and sometimes compressed between the bracket members, the rubber or other elastomer is adhesively bonded to the brackets.
The bonding requirement in such an application can vary from structural to nonstructural. In structural bonding, where the bond is expected to sustain a substantial load, the bond is considered successful if the entire bracket or substrate is covered with torn rubber after failure of the test specimen. In nonstructural bonding, the rubber-bracket interface is not subjected to large tensile or shear loads. It is only necessary to keep the rubber in intimate contact with the bracket. The bracket is usually, but not necessarily always, steel or aluminum.
The techniques employed for such rubber bonding are divided naturally depending on whether the bond is made while the rubber cures, in situ bonding, or after cure, post-vulcanization bonding. In situ bonding is the accepted method for the manufacture of many natural or synthetic polyisoprene rubber bonded articles such as mounting devices where a rigid insert, commonly a steel tube, is substantially surrounded by a body of rubber. An adhesive is first coated on the rigid insert from a solvent or water carrier and then dried. The insert is then placed like a core member in the rubber mold prior to injection of the uncured rubber. Adhesive cure takes place during the rubber curing process. Examples of suitable adhesives for in situ or pre-vulcanization are the reactive elastomeric products sold under the trade names of Chemlok(trademark) and Thixon(trademark), respectively, by Lord Corporation and Morton International in the United States. Such in situ bonds are usually stronger than post-vulcanization bonds.
A number of techniques are used for post-vulcanization bonding. A most common practice utilizes the same type of reactive elastomeric adhesive used for in situ bonding. In this case, the cured rubber mass is held in contact with the adhesive coated surface and heated. Substantial pressure is required, often requiring the rubber to be compressed by about 20% of its original height. This method is particularly attractive for products such as bonded bushings where a cylindrical mass of rubber is compressed within an annular outer shell. The pressure requirement is easily met by the rubber being captured within the outer shell.
The use of epoxy resin in the manufacture of vehicular powertrain mounts was taught as an alternative to reactive elastomeric adhesives for post-vulcanization bonding in U.S. Pat. Nos. 4,987,679 and 5,031,873, assigned to the assignee of this invention. This process utilizes a two-component epoxy adhesive to bond cured rubber to rigid inserts. The primary advantage of the epoxy adhesive over conventional post-vulcanization bonding using reactive elastomeric adhesives is that pressure is not required to achieve good bonds. Also, a fair amount of mismatch between the rubber and the rigid insert can be tolerated since the mixed but uncured epoxy is mobile and fills gaps and still bonds well. This technology has made it attractive to convert designs that would otherwise be bonded in situ. It is not necessarily attractive for applications such as bushings where the rubber mass must be pushed into a constrictive shell. The uncured epoxy on the bond surface of the shell tends to be wiped out during rubber insertion, resulting in weak bonds.
Several production powertrain mounts are currently manufactured utilizing such two-part epoxy adhesives. In these applications, an electrophoretically-deposited cathodic resin is used on the surfaces of the metal bracket for the dual purpose of providing a primer for the epoxy adhesive as well as providing required corrosion protection in areas not bonded. The cathodic primer is usually applied over a zinc phosphate coating (actually a mixed zinc-iron phosphate) integral with the surface of the steel bracket.
The cathodic, electrophoretically-deposited coat is actually a single epoxy resin component paint which is electrolytically deposited from an aqueous bath. After the coating application or electroplating of the cathodic electrophoretic epoxy paint, the coated metal parts are cured at temperatures of 350xc2x0 F. to 450xc2x0 F. to convert the epoxy coating into a tough chemical and environmentally-resistant coating. In other words, the coating is cured or crosslinked. Such coatings are now used widely in the production of automotive bodies where the entire body is dipped into a tank and primed as a unit. Exemplary electrophoretically-applied epoxy paints are manufactured and sold by companies such as PPG under trade names such as Powercron 500(trademark) and Powercron 640(trademark). Electrophoretically-deposited epoxy paints are baked after application at temperatures of the order of 400xc2x0 F. until they are cured. In their baked condition, they are scratch resistant and resistant to solvents such as gasoline or automobile oils. In the case of body parts, uncured paints are sprayed onto the primed surface and later baked to dry the paints. In the case of the above-mentioned engine or transmission mount applications (i.e., the ""679 and ""873 patent disclosures), a two-part epoxy adhesive is applied on top of the epoxy prime coat for the purpose of bonding the rubber-cushion body to the primed metal surface.
It is, of course, always of interest to simplify and render less complicated and expensive the practice of bonding a cured rubber body to a support bracket in a vehicle mount application and in other applications.
This invention is based on the discovery that it is possible to eliminate the epoxy adhesive as described in the above ""679 and ""873 patents and bond vulcanized polyisoprene rubber directly to a baked electrophoretically-applied epoxy prime coat material. In a more general statement of the invention, it has been found that it is possible by application of suitable pressure and heat to bond cured rubber containing 40% by weight or more natural or synthetic polyisoprene to a baked or cured epoxy resin-coated mounting device surface. This results in an excellent bond between the bulk elastomer and a bracket member which is capable of sustaining the loads that are common in vehicle mount applications and the like.
A preferred application of the invention is the bonding of natural rubber to an electrophoretically-applied epoxy resin prime-coated bracket. After the prime-coated bracket has been baked, for example, at a temperature of 350xc2x0 F. to 450xc2x0 F., to convert the coating into a tough, chemical- and environmentally-resistant coating, the bracket is ready to serve as a bonding surface for the resilient natural rubber body. The surface of the rubber body is chlorinated by immersing the bulk rubber in, for example, an aqueous solution of acidified sodium hypochlorite. The chlorinated surface rubber body is then pressed against the baked epoxy prime coat and the assembly heated to a temperature of the order of 250xc2x0 F. to 350xc2x0 F. for 15 minutes or so to form a strong bond between the chlorinated natural rubber surface and the epoxy prime coat.
As will be discussed below, this practice may be utilized with other suitable bulk resilient polyisoprene-containing elastomeric bodies and other suitable pre-cured epoxy paint coatings. Examples of such other paints include the electrostatically applied powder epoxy paints and other one-component (as opposed to two component adhesives) epoxy paint, or paint-like, resins. However, the common, surprising and inventive feature is that such elastomeric bodies can be urged under pressure against such cured epoxy resin surfaces and heated to a suitable elevated temperature for the purpose of effecting a strong bond between the epoxy-coated bracket and the bulk resilient elastomeric body.
A remarkable aspect of this invention is that a strong bond is obtained between a suitable bulk resilient rubber-containing body and a cured epoxy resin paint. For example, baked electrophoretically-applied prime coats (E-coat) typically exceed a 2H minimum hardness level in the ASTM D3363-92A Pencil Hardness test and a minimum of 60 inch/lbs in the ASTM D2794 Direct Impact test. Moreover, some users of Powercron 500 subject that baked E-coat paint to a solvent rub test. A suitably baked and cured Powercron 500 coating is required to withstand 50 back and forth rubs of a rag wetted with methyl ethyl ketone with no softening, marring or transfer of the paint to the rag. Baked E-coat paint films displaying such properties or characteristics would seem to be fully cured or crosslinked, and yet they participate in strong adhesive bond formation with a natural rubber body as described above.
Obviously, if a strong adhesive bond can be formed between a rubber body and a thus-cured epoxy paint layer, strong bonds can also be formed with one-component epoxy paint or other epoxy resin layers in a lower cure condition. Since the degree of cure of a generally solid, immobile one-component paint film is not easy to quantify, this invention is not limited by a state of cure reflected by the above hardness level, impact resistance and solvent resistance. In general terms, this invention does include the formation of an adhesive bond in a vehicle mount between a resilient elastomeric body and an epoxy resin layer that is substantially immobile at normal room temperature and thus apparently cured.
This practice finds useful application in the manufacture of transmission mounts, engine mounts or bushings, suspension mounts and bushings and other like vehicle mounting structures in which a bulk elastomeric body is sandwiched between two brackets, typically (but not necessarily) steel or aluminum brackets, and bonded to a cured one component epoxy resin paint coating on the bracket. The method is suitably applicable, for example, to natural and synthetic polyisoprene rubber, neoprene rubber and mixtures of 40 weight percent or more of such rubbers with other synthetic elastomers such as styrene acrylonitrile rubber, styrene isoprene rubber and the like.
Other objects and advantages of the invention will become more apparent from a detailed description thereof which follows.